1. Field of the Invention
The present invention relates to radios for use in vehicles, and, more particularly, to maintaining continuity of the radio listening experience in vehicles.
2. Description of the Related Art
The TiVo® concept of time shift operation has been applied to vehicle head units for all sources. This technique allows a listener to go back in temporal time in order to listen to broadcast segments that the listener missed due to stopping for gas or a telephone call, for example.
In North America, the high power transmitters have a fixed broadcast coverage area. That is, a user listening to a station cannot continue to listen to the station after he or she has driven out of the station's broadcast coverage area. This applies for both AM and FM broadcast bands inclusive of HD IBOC (in-band on-channel). Currently there is no means to achieve service following, in other words, continuous national coverage, for an analog/digital broadcast station in North America.
Radio head units are known to store audio and meta data to a nonvolatile memory such as a hard disk or SRAM for time shift operation. A disadvantage of this method is the high cost of the memory required in the head unit to store sixty minutes of time shifting. Another disadvantage is that storing and retrieving content from the hard drive or flash results in wear and early failure of these components. Another disadvantage is that digital audio sources employ compressed audio for transmission. For example, digital audio broadcasting (DAB) uses MPEG audio compression methods; and HD Radio uses MPEG SBR. Thus, there is a design tradeoff between whether to store the decoded data audio for time shifting, or store the raw compressed audio stream for time shifting and decompress the compressed audio when the user triggers a time shift operation. Both methods entail design complexity, CPU loading, and additional costs to design for storage.
Instead of time shift operation, broadcast following operation may be realized in digital audio broadcasting (DAB), which is digital. If digital reception is lost, then there is fallback to the FM analog band for what is termed DAB FM Service Link. The assumption is that there is simulcast audio on both DAB and FM (analog) frequencies and that broadcast following operation can be achieved in Europe and Rest Of World that adopt DAB broadcasting.
Broadcast following operation may also be realized in HD IBOC digital broadcast. When the HD IBOC signal is lost on the main program service, there is HD FM Blending to the FM analog fall back to achieve continuity operation.
The known methods do not address the problem of stations which do not support HD IBOC, such as pure analog FM stations, do not have broadcast continuity when the vehicle head unit travels beyond the transmitter broadcast span. The market scope of this is North America and Mexico where HD IBOC is prevalent.
The known methods also do not address the problem of HD IBOC stations which operate in the SPS (not main channel), i.e., on the side bands per the OFDM modulation of HD IBOC. Such stations do not have a fall back method to the analog station frequency in case the signal is lost, leading to loss of audio. The market scope of this is North America and Mexico where HD IBOC is prevalent.
The known methods also do not address the further problem of DAB stations with no hard link. FM analog simulcast information requires mute of audio when digital broadcast is lost. The market scope of this is Europe and Rest Of World (excluding North America and Mexico).
Pure on board implementation entails warranty costs in the case where storage is on hard disk drives. Pure on board implementation also increases material costs for the hard disk and SRAM and drives up design complexity costs.
On the other hand, a problem with pure off board implementation, assuming that the car head unit has access to the off-board services with the use of an embedded modem or cell-phone link, is that it is based on the subscription plan, and thus consumes minutes of the subscription plan with associated monetary costs.